Page 230 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Measurand Modulation

Measurand Modulation 223

10.5 Electromodulation Analysis
In a limited number of situations the absorption strength of an unknown liquid
sample can really be modified, and this forms the basis of an attractive detec-
tion scheme. Pesticides and herbicides that find their way into drinking water
supplies can in principle be quantified using the strong ultraviolet absorption
of these organic chemicals. However, the very low concentrations which are
of interest (ªppb) give only very weak absorption, in a drinking water matrix
with its own, much stronger absorptions due to unimportant dissolved inor-
ganic and vegetable matter, whose magnitudes also change from hour to hour.
We need to make the reference and sample measurements with as little delay
as possible, and with as little change to the mechanical configuration as possi-
ble. In this case it is a big help to make a difference measurement using the
sample itself as reference, as shown in Fig. 10.10. This is an example of a
stopped-flow system. First a UV/visible spectral measurement is made of the
water sample as received, for example pumped in from a fast-flowing sample
line but stationary during the spectral measurement. Then an intense flash of
light from a xenon flash tube illuminates the sample that is about to be
measured. Several herbicides can be rapidly photolytically degraded in this
way, while leaving the remainder of the water matrix absorbance spectrum
intact. We measure the spectrum once again, and then calculate the difference
in software before pumping in a new sample. This difference spectrum is
characteristic of the herbicide present, with the bulk of the time-variable
matrix absorption ignored. In some cases, for example the 292nm absorption
feature of alkaline chlorine disinfectants (HOCl) in drinking water, the charac-

Flash-lamps
Peristaltic Optical
pump source

Sample
inlet Detection system
(photometer)

Absorbance Sample spectrum (l1)
(before)
Absorbance
}
(after)

Difference l1 Wavelength Time
spectrum
Figure 10.10 In-situ flash photolysis is a technique to modulate the optical absorp-
tion of a photo-active species in a complex nonactive matrix. Subtraction of the
before/after spectra reduces the effects of absorbance drifts due to the matrix.

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